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This map shows the likelihood that rainfall from tropical storm Hilary will exceed the National Weather Service’s flash flood guidance over the 24-hour period starting 5 a.m. Monday, Aug. 21. The data used for this map was issued by the NWS on Monday at 1:17 a.m. The service calls this outlook its “best attempt to forecast flash flooding from heavy rainfall.”
What risk categories mean
This is how the the NWS Weather Prediction Center explains the categorizes in uses in its excessive rainfall forecast.
A marginal risk means that “isolated flash floods are possible.” The floods would be localized and primarily affect places that can experience rapid runoff with heavy rainfall.
A slight risk means “scattered flash floods are possible.” Under this category, floods are mainly localized, and most vulnerable are urban areas, roads, small streams and washes. Isolated significant flash floods are possible.
A moderate risk means “numerous flash floods are likely.” Many streams may flood, potentially affecting larger rivers. Expect “numerous flash flooding events,” with the possibility of “significant” events.
A high risk means widespread flash floods are expected. Areas that don’t normally experience flash flooding, could. Severe, widespread flash flooding could put lives and property in greater danger.